The invention relates to absorbent paper products and, more specifically, to those made of cellulose wadding for sanitary or household use. It applies basically to toilet paper, though it is optionally applicable also to such household papers as paper towels, napkins or handkerchiefs.
In general, such papers consist of one or more cellulose wadding plies or superposed layers. The plies can be bonded to each other. In the case of toilet paper, conventional products comprise two, three or even four assembled plies.
More specifically, the invention concerns a product of at least three plies and its method of manufacture.
U.S. Pat. No. 3,414,459 describes an absorbent paper structure including at least two embossed plies. Mention is made therein of a product of three assembled plies, a third ply being sandwiched between two external embossed plies. This structure is based on the plies each having a specific surface weight of about 17 g/cm2. In the manufacturing method of this patent, the two external plies are separately embossed with a salient pattern consisting of discrete protrusions. Each of the plies is made to pass between a saliently engraved metal cylinder forming the desired pattern and a smooth rubber cylinder. Each ply so made generally has a pattern pitch of 3 to 30 protrusions or salient tips per unit area, here cm2. In the text below, the expression xe2x80x9cpattern densityxe2x80x9d denotes the number of protrusions per unit area. As regards the three-ply structures illustrated in the above patent, the pattern density is about 11 protrusions/cm2. The unembossed third ply is sandwiched between the two embossed plies and, more specifically, is placed between the distal surfaces or peaks of each ply. In other words, the protrusions point to the inside of the structure. The three plies are assembled by being made to pass through the clamping gap present between two metal cylinders, each engraved with embossing means, for the external plies. The salient elements of each engraved cylinder are mutually opposite. This combining technique is generally called xe2x80x9ctip-to-tipxe2x80x9d. In the case of the above patent, the three plies are mutually bonded by applying a sufficiently high pressure to produce interfiber links. No adhesive is applied to the distal pattern surfaces of the protrusions of the external plies.
The manufacturing method described in the prior patent entails rapid wear of the embossing cylinders because of the high clamping pressure required to assemble the three plies. To the knowledge of applicant, this method is not used in industry. Lastly, the structure manifests low pattern density (about 10-15 protrusions/cm2). The triple-ply products with such a structure are mainly used as household paper towels for which the main desired properties are strength and, especially, absorption. On this account, the embossing patterns are comparatively deep and hence of low density.
Other absorbent paper products constituted of three plies can be made using different assemblies. Using two distinct embossing apparatus, on one hand a set of two superposed plies is embossed and on the other hand a third ply is embossed. The first two superposed plies as well as the third ply are embossed with salient patterns consisting of protrusions. The pattern density perforce is low, namely less than 20 protrusions/cm2 for this sort of product. The two superposed plies and the third ply are assembled using a mating cylinder so that the distal protrusion surfaces of the two superposed plies are opposite the surfaces situated between two protrusions in the plane of the third ply. This technique is called xe2x80x9cnestedxe2x80x9d.
A method for combining three plies is described in European Application No. 0,570,578. This method consists in separately embossing a first and second web itself consisting of the superposition of two layers using a first and second embossing means including an embossing pattern having a density less than 20 protrusions/cm2. Then, depositing at least in part an adhesive on the peaks of the protrusions formed in the first or second web and assembling the two webs using a non-yielding, mating cylinder running parallel to one of the engraved cylinders of an embossing means in such a manner that their protrusions mutually nest. In this method, the mating cylinder is kept as a stop at a predetermined minimum distance from the engraved cylinder.
Thick products with a good feel are obtained from such products. However, especially as regards toilet paper, softness is less than optimal because of the comparatively coarse embossing. Also, strong embossing is required to arrive at the desired substantial thickness.
Further compound products consisting of three plies have been disclosed. Illustratively, European Application No. 0,564,319 describes a product having three plies that were embossed separately and assembled consecutively. More specifically, small and large protrusions are embossed into a first ply and protrusions are embossed in a second ply. These protrusions are separated from each other by the same distance as the small protrusions of the first ply and their height is such that the sum of their height and that of a small protrusion of the first ply is equal at least to the height of a large protrusion of the first ply. The protrusion ends of the second ply are coated with adhesive and then a pad is created between the first and second plies by superposing the small protrusion ends of the first ply with the protrusion ends of the second ply in tip-to-tip positions. Moreover, a third ply is embossed separately at a pitch identical with that of the large protrusions of the first ply. The protrusion ends of the third ply also are coated with adhesive and this third ply is assembled with the pad formed between the first two plies in such a manner that the protrusion ends of the third ply bond in tip-to-tip manner with the second ply near the large protrusions of the first ply.
The product so made offers improved thickness and rigidity but its structure is comparatively complex, requiring several levels of bonding. The method itself is laborious and entails relatively complex industrial apparatus.
This method allows only manufacture of embossed products with pattern densities less than 20 protrusions/cm2. Again, the thickness of products so made and consisting of three plies is not optimal.
The object of the invention is to palliate the above noted drawbacks regarding both the products and their method of manufacture.
Another object of the invention is to provide a novel product including at least three plies which are both thick and soft and have a good touch, as well as further being crush-resistant when wound in a roll, thereby offering a combination of properties absent from existing products.
Still another object of the invention is a novel product of at least three plies of the point-to-point type, useful as toilet paper.
The object of the invention is an absorbent paper product where this product has an approximate specific surface weight of 36 to 105 g/m2 and includes at least three plies, namely external first and second embossed plies each including salient patterns having at least in part discrete protrusions pointing inward of the structure, and a central unembossed ply. The plies are assembled by bonding the distal surfaces of at least a portion of the protrusions of each of the external plies to the central ply.
In an essential feature of the invention, each of the embossed external plies is adhesively bonded to the central ply and at least one of the external plies has a pattern density exceeding 30 protrusions/cm2.
In a surprising manner, such a structure provides both softness and thickness. The fine embossing on one hand imparts great softness to the outer web surface and on the other hand provides good crushing strength. The latter property is enhanced by the central ply precluding any mutual nesting of the two external plies that would take place in the absence of the third ply because of the low support surface of such fine protrusions.
In an advantageous feature of the invention, each of the external plies has a pattern density less than 90 protrusions/cm2.
In another feature of the invention, the pattern density of the external plies varies from one to the other. In still another advantageous feature, the two different patterns allow partial joining, that is, the number of bonding sites is less than the number of protrusions contacting the central ply. This partial bonding makes the web more pliant. In particular, one of the plies includes combined patterns, that is two different patterns, one of which is the background pattern. In particular, the two combined patterns can have different elevations relative to the ply""s reference plane.
In still another feature of the invention, the fibrous and/or chemical composition of the central ply differs from each of the compositions of the external embossed plies.
Another object of the invention is a method for manufacturing an absorbent paper product including at least three bonded plies, each ply having a specific surface weight of about 12 to 35 g/m2.
In another advantageous feature of the invention, the delamination strength between one of the external plies and the central ply is substantially different from that between the other external ply and the central ply.
In one feature of the invention, the method of the invention allows in particular the making of a product having a differential delamination strength involving:
embossing a first ply and a second ply with salient patterns of discrete protrusions,
superposing a third unembossed ply on the protrusions of the first of the embossed plies,
depositing an adhesive in particular on the third ply, and
assembling the superposed first embossed ply and the third unembossed ply to the second embossed ply in such a manner that the distal protrusion surfaces of each of the first and second embossed plies are at least partly mutually opposite, and applying sufficient pressure to ensure bonding of the three plies by means of the adhesive.
In an advantageous feature of the invention, an aqueous adhesive is deposited to the outer surface of the third ply opposite the distal protrusion surfaces of the first ply.
In another feature of the invention, an aqueous adhesive is deposited with sufficient energy by atomization on the third ply in order that part of the adhesive crosses the third ply and allows subsequent bonding of the three plies to each other.